This invention relates to the separation of individual isomers from a mixture of isomers by a process of selectively melting the isomers in the presence of an immiscible liquid. More particularly this invention relates to the separation of para-, meta-, and ortho-xylenes by a process of selectively melting the isomers in the presence of an immiscible liquid comprising a water, a lower alkanol, and a lower glycol mixture.
In recent years there has been a growing need for feedstocks rich in each of the isomers of xylene. Feedstocks rich in para-xylene, for example, are needed for the production of the terephthalic acid; those rich in meta-xylene, for the production of isophthalic acid; and those rich in ortho-xylene, for the production of phthalic anhydride. These products are important commercially and are used for the production of polyester fibers, alkyd resins, protective coatings, and the like.
The separation of xylene isomers by fractional distillation is extremely difficult because of the small differences in their boiling points as shown in the following table:
m.p. b.p./760mm ______________________________________ Ethylbenzene -95.degree.C 136.degree.C Para-xylene 13.3 138.3 Meta-xylene -47.9 139.1 Ortho-xylene -25.2 144.4 ______________________________________
On the other hand, the differences in melting points are relatively large and, as a consequence, several processes have been developed utilizing fractional crystallization. In particular, crystallization processes for the separation of para-xylene are well known and are practiced on a large commercial scale. One of the serious drawbacks of such processes is that the amount of para-xylene that can be recovered is limited by the crystallization threshold of the other xylene isomers; that is, eutectic solutions are formed which upon further cooling yield solids of more than one isomer. Not only does this property limit recovery of one isomer but it also prevents the separation of a second isomer in its pure form. The state of the art is further illustrated by U.S. Pat. No. 3,798,282 which discloses the separation of high-purity metaxylene from a mixture of meta and para crystals by crystal size classification. U.S. Pat. No. 2,769,852 is directed primarily to the separation of para-xylene by crystallization of a xylene concentrate in methanol-water mixtures. U.S. Pat. No. 3,758,601 is also primarily concerned with the separation of para-xylene from a hydrocarbon mixture by crystallization in a water-methanol-glycol mixture. Examples of other patents teaching fractional crystallization techniques are U.S. Pat. No. 2,724,007; U.S. Pat. No. 3,643,453; and U.S. Pat. No. 3,825,614. All of these patents utilize crystallization as a technique for isolating a single isomer, specifically either para-xylene or meta-xylene. None of these patents discloses or suggests a process of selective melting in the presence of an immiscible liquid for separation of all three xylene isomers.